6. Exploitation

6.1 General
6.2 Surface water conservation
6.3 Groundwater exploitation
6.4 Water lifting devices

6.1 General

The exploitation of an available water resource must always be closely linked to the ecology of the available pasture and to its actual or potential carrying capacity in terms of human and livestock units. Overdevelopment is not only wasteful of scarce resources, but it also carries the potential danger that, by attracting increased numbers of livestock without adequate control, the available forage is overutilised to its eventual destruction. This applies particularly to the fragile ecology of arid and semi-arid zones.

Under higher rainfall conditions with more abundant vegetation, overdevelopment in providing a more-than-adequate water supply is not so serious, and may indeed be desirable if one bears in mind that good range management can increase the carrying capacity significantly. A concomitant increase in the human population will lead to the establishment of new settlements and their infrastructure, creating a water demand which can often be met from the initial installation.

The simplest form of water exploitation is to water stock at the river bank or lake shore. All other forms of water exploitation require the use of technology which must be appropriate to the local conditions. Whatever its nature, an installation must be technically sound and reliable in operation; its capital and operating costs must be commensurate with the benefits it provides; and it must be capable of operating continuously with maintenance services which can largely be provided locally without undue reliance on assistance from a remote base.

While the criterion of sound engineering is readily understood, the other two criteria merit explanation. In the livestock industry, the inputs of development such as fencing, dips, skilled supervision and not least water, all require cost inputs which must be related to the outputs i.e. more and better animals and animal products. If the inputs exceed the outputs, the undertaking is not viable and, unless it is accepted as a social need to develop a particular livestock area, the project should not start. If the social need dictates the establishment of a project, then an element of subsidy must be built into the financing programme from the beginning.

The cost of water sufficient to sustain an animal for 1 year can be calculated from the designed capital cost and the estimated annual cost of operating the supply. This is very often a crucial factor in the economics of the entire undertaking. While economies can be achieved in such items as fencing, e.g. by using natural boundaries and supervising stock movement from paddock to paddock more closely, there are distinct limits below which economies in the cost of a water supply result in poor performance and unreliability.

The cost of operating and maintaining a supply is an item of vital importance, and yet it is overlooked or ignored all too frequently when capital development for all types of water supply is being planned. All water supplies require some maintenance; even a small earth dam, from which the stock are allowed to drink direct, needs care to ensure that the wall is not trampled and eroded by uncontrolled animals, and that the spillway is kept clear and safe from erosion. If cattle troughs are provided downstream, the control valves must be checked and cleaned periodically.

Skilled maintenance is required when machinery is used to extract and convey water to cattle troughs. Pumps and engines wear out and require a supply of spare parts, in addition to the daily amount of fuel and lubricants.

Such spare parts must be readily available, either on the project or from a local supplier. Skilled staff are needed to carry out repairs and to fit replacements. The pump attendant has to be taught how to look after the machinery in his charge and must be closely supervised to ensure that he does his duty. If skilled mechanics are not available within reasonable distance of the installation, it will be necessary to employ such a man on the spot. The timely delivery of fuel is also a very important organisational aspect. All these items cost money, which must be provided for in the annual budget if the supply is to function reliably and contribute to the welfare of the animals and their owners. Wherever possible, the design of an installation must be suited to the local conditions as they affect operation, and this is particularly true for the remoter range areas where communications are poor or lacking and the technical ability of the people is low. If there is no simple alternative to a deep borehole with a costly pumping unit, for example, then adequate technical skill, spare parts and tools must be included in the overall plan and in the annual budget.

The choice of groundwater as a source for watering livestock is governed by the physical factors of its availability; by its location in relation to surface water sources; and by its quality and quantity. Economics also play an important role in decision-making since groundwater requires energy to bring it to the surface, in addition to that needed for distribution.

Despite this there are many situations where, because of propinquity and ease of access, groundwater is preferable to surface water, which may have to be piped or carried from a distant source. In many parts of the arid and semi-arid zones permanent sources of surface water do not exist, and groundwater then becomes the sole means of supplying herds and flocks.

The traditional grazing lands of tropical East Africa, for example, are characterised by the availability of forage and water, and their extent can be seen from a map of livestock populations, such as that in King (1983). Within these lands, many areas are underused because the natural supply of water does not match the forage potential. Others are overexploited or overgrazed because of the abundance of uncontrolled water supplies. It is these latter areas which are vulnerable to increasing desertification. As far as water is concerned, therefore, the task of range management is to apply controls in those areas where water is abundant. Such controls should be aimed at reducing the stock population to the carrying capacity of the pasture reserve, improving the quality of the animals produced and developing new areas of good forage potential. By providing additional water sources, areas can be used which are at present underused or not used at all. Such water sources should be strategically located and capable of effective management to ensure that the risks of overgrazing which occur in the natural state are minimised as far as possible.

Boreholes and wells equipped with pumping units are of special value as controls both in the grazing blocks and along stock routes, because they can be easily closed down simply by removing part of the equipment or by delaying delivery of fuel at times when the pasture has to be husbanded or fully rested for a period.

Careful planning is required, whether it be for the improvement of existing grazing areas, for the development of extensions or for totally new ranching blocks. Planning should be based on an assessment of the water resources, both surface and underground, and a review or a fresh survey of the range potential. This requires a knowledge of the potential water resources.

While surface water resources can often be assessed with relative ease, the identification of groundwater resources requires careful hydrogeological surveys to indicate favourable areas for development. Before a source can be positively included in the overall plan, wells need to be sunk or drilled and water test pumped.

Groundwater surveys should, therefore, precede the completion of the final plan for grazing blocks and cattle movement in the rotational pattern. This will avoid the mistakes that have occurred in the past due to plans for forage utilisation being based merely on assumptions of groundwater availability. When the groundwater survey has indicated that water is either inadequate or is to be found elsewhere, substantial revisions of block boundaries and grazing plans have resulted.

While this risk is not great in areas of sedimentary formations with a continuous aquifer, it is very real in those parts of the arid and semi-arid zones which are underlain by crystalline rocks of the Precambrian, or by Tertiary or older volcanics with discontinuous aquifers.

6.2 Surface water conservation

6.2.1 Protection of springs
6.2.2 Rainfall harvesting
6.2.3 Water spreading
6.2.4 Tanks and hafirs
6.2.5 Small dams
6.2.6 Reservoirs

Excepting those narrow zones which are watered by perennial rivers, or permanent lakes and swamps which are the final recipients of catchment runoff, almost the entire tropical region is subject to periodic droughts of varying severity (see Table 16). Throughout the region there is a need to conserve the seasonal overland flow to ensure continuous water availability. This is especially so in the semi-arid and arid zones, where even in 'normal' years substantial runoff for short periods is followed by much longer periods of complete drought. In the humid and highland zones periods when no runoff occurs and when the flow in streams ceases or diminishes are shorter, but conservation measures to hold back the water and thus even out the flow pattern are still necessary for optimum water development and exploitation.

Table 16. Summary of main areas with known large percentage of less than normal annual rainfall.

Good water conservation depends on good soil conservation, and the two are inseparable. Soil is conserved naturally by vegetation but, where this is disturbed by bad farming practices or by overgrazing, runoff will be intensified and there will be less recharge to soil moisture storage which sustains the vegetation. Intensified runoff erodes the soil and destroys its structure so that during the dry periods more soil is removed by wind. Successive runoff periods cause sheet and gully erosion even on gently sloping ground. Surface sealing leads to increasing velocities and the runoff periods become shorter but more intense, a process which leads to accelerated desertification.

If the soil cannot be conserved by natural means, artificial methods must be applied. In the more humid areas these may comprise trash lines on cultivated ground, earth embankments and terraces, and even stone walls. In the range areas of the semi-arid zone, a cheap method is the establishment of contour strips of natural vegetation (bushes and grasses which are not grazed), which will do much to retard the runoff and sustain the usable pasture. Badly degraded areas may require more extensive soil conservation schemes before surface water can be used as a resource. Although these are costly, it cannot be overemphasised that investment in the surface water conservation techniques outlined below will be wasted if due regard is not paid to reducing the velocities of surface runoff and the rate of soil removal.

6.2.1 Protection of springs

A spring is a concentrated discharge of groundwater appearing at the ground surface as a current of flowing water. To be distinguished from sponge are seepage areas which indicate a slower movement of groundwater to the surface.

The quantity of water from a spring can be substantially increased by digging out the area around the spring down to an impervious layer, to remove silt, decomposed rock and other rock fragments, and mineral matter sometimes deposited by the emerging groundwater. In doing this particular care should be taken, especially in fissured limestone areas, to avoid disturbing underground formations to the extent that the spring is deflected in another direction or into other fissures.

The essential techniques for protecting a spring are as follows:

i) preventing overland flow from contaminating the source by digging a drainage ditch above the spring to divert surface runoff;

ii) constructing a simple collecting structure or reservoir around the spring to increase its yield during the day; and

iii) providing an outlet pipe to a discharge point or cattle trough to prevent contamination or destruction of the reservoir.

Since the major use of any watering point is during the day, constructing a reservoir will allow the spring to recharge the water consumed, enabling the maximum use of even quite small springs.

6.2.2 Rainfall harvesting

Rainfall harvesting is the term given to the conservation and storage of rainfall on outcrops and sheets of bare rock. In many parts of the tropical pastoral regions there exist inselbergs of granite or granitoid gneiss, which stand out prominently from the surrounding peneplains and receive additional precipitation from orographic rainfall, dew and mist. By encircling such rocks with a low masonry wall the runoff from the rock catchment can be guided into a storage area, which may be a natural fold in the rock blocked off with a concrete wall or, if such is not present, a tank excavated at the base of the rock. Sixty-three rock catchments and storage tanks were successfully constructed in Kenya between 1945 and 1962 as part of the surface water conservation programme of the Ministry of Agriculture (Min. of Agric., 1962).

The soil at such rock bases is usually very sandy and permeable. It is then necessary to line the excavation with masonry, concrete or brickwork, or with an impervious membrane such as sheet of butyl rubber or polythene. To prevent evaporation, pillow tanks of butyl rubber (which are made in capacities of up to 4500 m³) have proved very effective in Kenya and their cost is generally below that of concrete or masonry work. Even in the arid zone with an annual rainfall of less than 250 mm more water runs off the rock during a short storm of shower than can be effectively and economically trapped and stored.

Disturbed and fissured rock outcrops with numerous pockets of vegetation, while still receiving the same volume of precipitation, would not normally be considered for development because of the additional cost of bush clearing and sealing cracks and fissures. Their potential yield of water for stock is lost to deep infiltration and to evapotranspiration.

Often the catchment needs modification, usually by making the soil surface more impermeable to increase the amount of runoff. There are several methods available e.g. land alteration, chemical treatment and soil covers. Currently rainfall harvesting is for small-scale use, for farms, villages and livestock. Because rainwater harvesting depends on natural rainfall, the system will fail in drought years unless there is adequate storage. A rainfall-harvesting system once installed, however, will provide water without requiring fuel or power.

Under favourable conditions land alteration methods are the least expensive of all. Arid developing countries that produce and refine crude oil could use asphalt to construct harvesting catchments. Heavy petroleum fractions such as asphalt have limited demand and are often persistent pollutants, difficult to dispose of. Chemical treatments and soil covers, though still mainly experimental, are used worldwide on a modest scale. Although such treatments are technically feasible and successful, they are not yet economically attractive enough to generate widespread adoption (National Academy of Sciences, 1974; Wenner, 1973; FAO 1976).

No method of rainwater harvesting has been subjected to a long-term economic analysis. Large field trials in different areas are needed to build up a data base that could lead to a better understanding of the economic viability of alternative methods in varying economic environments. The major technical need is to reduce costs of sealing catchment soils and to make the treatment practical for a wider variety of soils and situations.

In applying water-harvesting methods to a given area, care is needed to minimise side effects. Poorly designed or poorly managed rainwater harvesting can lead to soil erosion, soil instability and local floods. Soil erosion, a constant concern, can be controlled if the slope is short and not too steep (and if land drains are suitably sloped). Slope also affects the quality and quantity of runoff. The most efficient water harvest is from small, gently sloping (preferably 1 to 5%) catchments.

6.2.3 Water spreading

One of the simplest techniques of using surface water for forage production and livestock water supplies is to take off streamflow from a suitable site by means of a diversion weir or an abstraction ditch and to spread it over the flood plain with a series of bunds, the last one leading into a small-stock watering dam or waterhole. It is to be stressed that such systems are difficult to maintain in remote areas of Africa and, if once allowed to deteriorate, may cause more damage to the land than if they had not been built. By retarding runoff more water is retained in the soil, and surplus flowing water is relatively silt-free.

Figure 24 gives two examples of layouts for water spreading (Pratt and Gwynne, 1967). In both examples a diversion furrow is set to take a proportion of stormflow from a river, and to guide this water far enough from the river to allow free flooding. This should only take place, of course, if the gradients on the flooded area are gentle enough to prevent erosion.

A disadvantage of this type of scheme is that the main offtake and diversion bunds can rarely deal with the exceptional flood. If such a flood occurs, widespread damage may ensue which will require repairing before the next flood comes. A succession of high floods may cause such widespread damage that the scheme becomes unacceptable to pastoralists who, in any case, may be reluctant to accept this type of sedentary commitment.

6.2.4 Tanks and hafirs

In addition to rainfall harvesting, water also has to be stored at or near points of use such as the centre of a grazing paddock or block or at temporary settlements and villages. Such storages do not normally exceed 90 m³ in capacity, while a capacity of 45 m³ is more common. Storage is necessary to ensure at least a 2 days' supply in the event of a mechanical breakdown interrupting the flow from the source.

A good form of storage, much used in East Africa and elsewhere, is the van Meerten tank, which is a circular reservoir of blockwork having a capacity of 45 m³. It is built integrally with a circumferential cattle trough into which flow is controlled by a ball valve housed in a small chamber. A short pipeline serves a number of stand pipes for use by humans (Ministry of Agriculture, 1962).

Figure 24. Examples of layouts for water spreading.

Other types of diurnal storage are tanks made of corrugated, galvanized iron sheets or steel panels, circular butyl tanks held in a metal framework, or simply excavations in the ground. The latter, provided they are watertight and sited on a slope sufficient to permit a draw-off pipe to be connected to the base and taken out in a trench, are simple to construct and relatively cheap.

In a flat terrain, where drainage ways are absent or nearly so, it is still possible to conserve and store the runoff by excavating reservoirs, which have a variety of names. They are frequently referred to as 'tanks' but are also known, for instance, as hafirs in Arabia and East African countries, and chacos in the Americas. Figure 25 gives an example of a rectangular tank, many of which were constructed in East Africa using communal labour (Ministry of Agriculture, 1962).

If dug by hand, such reservoirs are usually rectangular in shape, but it has been found that an elliptical shape will make the most efficient use of earthmoving equipment. Runoff is guided into one end of the tank, with appropriate anti-erosion measures to protect the steep entry slope, while the other end is suitably graded, protected, and used as a cattle ramp down to the water. The spoil removed from the excavation should be carefully placed on two or preferably three sides of the reservoir, to form an embankment which will act as a windbreak to reduce evaporation losses and to prevent uncontrolled entry of animals. Locally adaptable trees such as Commiphora, Euphorbia or other thorny shrubs, planted on the outside of the banks, will protect them from wandering animals and will form an additional windbreak.

Haphazard dumping of the excavated soil at some distance from the excavation, rather than placing it to form an embankment, should be discouraged because it then becomes more difficult to control thirsty animals and to prevent premature silting-up of the storage.

Runoff is guided into the tank by means of feed furrows extending outwards more or less along the contour, but at a gently rising gradient. The length of such furrows may be 1 km or more to embrace a sufficiently large catchment area. This area is determined by the potential available runoff in relation to the seasonal rainfall. The furrow gradient will vary with soil type, and should be such that the furrow will neither scour nor silt up too rapidly. Some silting in the furrow is preferable to scour, which carries undesirable silt into the tank. In sandy, loamy soils a gradient of 1 in 600 has been found suitable. It can be a little steeper in clay soils.

The furrow can be dug by hand or by light machinery such as a tractor-towed terracer or grader. It will be a shallow 'V'-shaped cut, with soil packed as a low bank on the downhill side. Such furrows are a good aid in soil conservation, as they act as a trap for grass seeds which germinate and help to bind the soil. When the furrow silts up after several seasons, and is well grassed, it should not be dug out but a new furrow should be made just above it.

The idea of a so-called 'disposable' pond or tank, i.e. a pond with no provisions for reducing the silting-up, has been suggested. Such a pond acts as a silt trap and, when full, it indicates that livestock should be moved to another locality where water and forage are available. The advantage of such a policy is that after the pond has silted up, the surrounding vegetation can rest and recuperate from the intensive grazing which occurs around a watering point. However, the disadvantages of such a policy are several. First, it is destructive in terms of sites for small tanks or ponds, and good sites may not be abundant. Second, the lifetime of such a pond may be only one season or part of a season, making it difficult to integrate water supply into a grazing scheme. Third, because of the difficulty in predicting the life of such a water supply, it can only be regarded as a supplementary measure. A permanent water point within a grazing block must be included in the range management plan to provide water in periods of severe drought.

In the moister arid zone and in the humid zone, where water supplies for livestock are part of the domestic supply for settlements and centres of human population, stock should not be allowed to drink water directly from a dam or tank. With a dam, pollution of the water can be prevented by fencing off the dam area and conveying the water through the draw-off pipe to cattle troughs outside the perimeter. With a tank, the water must be lifted to the cattle troughs by a mechanical device such as an engine-driven pump, a windmill or a hand pump. Alternatively, a human watering point supplying satisfactory water can be constructed by digging a shallow well and connecting it to the tank by an infiltration gallery filled with graded sand and gravel (Figure 26). Since tanks are dug in impervious soil to avoid seepage loss, direct infiltration to a well through the natural soil will not be satisfactory.

6.2.5 Small dams

Perhaps the most common form of water conservation structure is a small earth dam across a drainage way which may have permanent or seasonal flow. 'Small' in this context is used to describe a structure no more than 3 m high which can be constructed without professional engineering advice. Given that suitable dam-building materials exist, the capacity of the dam will be determined by a combination of factors beginning with the topographical shape of the valley. A narrow, steep-sided valley allows the design of a short dam wall of considerable depth and a small surface area of open water which will be less susceptible to evaporation losses. A wide flat valley calls for a long wall, which may be costly, and which will result in a large surface area of open water and high evaporation.

The desired capacity is one which meets the needs throughout the dry period between rainy seasons, with allowance being made for evaporation loss. It will be larger in a region of unimodal rainfall - since the dry periods are longer - than in one of bimodal rainfall. The size of the dam will also be affected by the catchment area and the available runoff, which is estimated from the prevailing annual rainfall and runoff characteristics (see Chapter 3). This knowledge is also necessary to compute the design flood, which dictates the size and capacity of the spillway to discharge surplus water after the dam has filled. In the case of small dams where no loss of life is at risk if the dam fails, it probably will be uneconomic to design for a flood with a return period longer than 10 years. Larger structures require a higher safety factor and a detailed economic analysis to decide upon the return period of the design flood.

In the arid zone, where annual rainfall is often erratic and where there are wide variations from the mean, a dam may not fill in one season. But given 1 year of good rainfall, it has been found that such a dam is likely to fill and remain full thereafter.

Figure 25. Rectangular tank or hafir with dimensions for different capacities.

For all sizes of tank, the recommended width of the shoulder between the tank and the outer retaining wall is 10 m, the length of the intake as illustrated is 60 m and its width 6 m. The levée can be any length.

Source: Pratt and Gwynne (1977).

In the humid zone and in the highlands, where rivers flow throughout the year but their volume of discharge may drop sharply during a dry period, regulatory storage dams, built to provide usable water and to maintain the river flow at the desired volume, must be fitted with draw-off pipes through the base of the wall which can be opened, to allow downstream users to receive their allotted share (compensation flow).

Figure 26. Well point used for clean water supply from stock pond.

All dams should be protected by at least a minimum of soil conservation upstream of the wall to reduce the rate of siltation. The minimum would be a diversion ditch starting from a point upstream of the top water level in the dam and running around the impoundment on graded slopes, eventually leading the water back into the channel below the dam wall. Additional terracing above the ditch would also be desirable.

The apparent simplicity of small earth dams has led to countless attempts by enthusiastic, but ill-informed, would-be builders. The large proportion of failures has shown that care in construction and knowledge of the basic principles of siting, designing and building are essential.

What are the reasons for failure? The washing away or breaking down of a dam is more commonly due to overtopping and erosion of the embankments rather than to insufficient strength of the dam to support the weight of water. Continued erosion will lead to the breaching of the dam wall and, often, total collapse. Widespread devastation can be caused by the sudden release of large quantities of water.

Apart from overtopping, which is usually due to inadequate spillways, earth dams may fail due to:

i) undermining, caused by water flowing below the embankment with consequent collapse of the material above;

ii) enlargement of fissures, caused by shrinkage or by the use of wrongly chosen or badly compacted materials;

iii) percolation along tree roots, which were not properly cleared before construction commenced or which were allowed to grow later; or

iv) general weaknesses caused by percolation through the dam.

Excessive percolation through the dam wall may lead to a build-up of pore pressure in the earth matrix and consequent slip or slumping. This can also be accelerated by other causes such as ant and termite activity and the action of burrowing animals.

Other factors having an effect on the efficiency and life of a dam are as follows. If there is an inadequate catchment area above the site, it will not fill. If the soil is of a porous nature, the water will soak into the ground and empty the reservoir. If the dam is not well sited, the stream supplying it may change its course or cut itself another channel. If the stream flow contains much silt and proper silt traps are not constructed, the dam may become useless in a few years due to silting.

In the preparation for the construction of a small earth dam, the following guidelines are suggested:

i) In planning an earthen structure sufficient time should be spent in the investigation and comparison of possible sites, in the gauging of streams, in the compilation of meteorological data and in surveys of the catchment area. Some estimate should be made of the likely sediment movement in the catchment.

ii) For every structure it is necessary to know the geological conditions of the site. The choice of the structure and its location depends on the foundation conditions and the treatment required for safety and leakage. On no account should any dam over 3 m in height be constructed without qualified advice on these aspects. Rock foundations usually pose no problem except that they may require grouting. Alluvial foundations may or may not be consolidated, and rock-filled or clay cores may need to be constructed by excavating down to the solid rock.

iii) The type of dam, its selection and method of construction, largely depends on the availability of construction materials. When taking materials from the bed of a stream, upstream or downstream of the dam, care has to be taken to ensure that this has no adverse effect on seepage or on the stability of the dam.

iv) With a concrete or masonry dam, water may be allowed to spill over the crest of the dam and flow over the downstream face in time of floods. In an earthen structure such spilling must never be allowed. This is a fundamental rule of such structures.

v) Spillways should be cut out of solid ground, clear of the dam itself. The spill way channels should be continued downstream well away from the 'heel', or the downstream edge of the base of the dam, in order to prevent flood water which has passed over the spillway from eroding the dam itself.

vi) In building up the dam, the material must be spread in continuous shallow layers 10 to 15 cm deep over the whole area. It must be kept damp but not too wet. Each layer must be well compacted before the next layer is added.

vii) The calculation of spillway size is one of the controversial subjects in dam design, and different methods of flood discharge prediction are referred to in Chapter 3. The information necessary for designing the spillway should always include information from people living in the area under study.

The selection of a design for earth dams depends on the foundation conditions and the types and quantity of construction materials available. Several excellent reference books are available such as: The design of small dams (US. Dept. Interior, 1965) and Manual of British water engineering practice. Vol. III (IWE, 1969). A more project-oriented approach will be found in Ahmad (1977).

Whereas small earth dams can be constructed without professional advice, and are suitable projects for self-help or community participation, due regard must nevertheless be paid to safety both during and after construction. During construction an adequate diversion channel must be provided to carry the design flood. In the semiarid regions, where a long dry season may allow enough time for the whole project to be completed within that season, this provision may be unnecessary. On the other hand, an untimely flood may carry away the whole of a partially completed earth dam.

Finally, there should be adequate inspection and maintenance facilities to prevent gradual erosion of spillways and embankments. If the slopes of the embankment are grassed to protect them from erosion by rainfall, livestock should if possible be prevented from grazing.

6.2.6 Reservoirs

A common design problem for all water supply systems is how to overcome the fluctuation in supply and demand. A stream which carried little or no water during the dry season can retain excess water from periods of high flow by means of storage or conservation reservoirs. The primary function of a reservoir, therefore, is to provide storage sufficient to meet the demand requirements. In the case of pastoral regions, demand for domestic and livestock purposes is not usually large compared to the storage capacity. The high evaporation losses from open-water surfaces (commonly 2 m or more per annum) and the percolation losses from simple earth dams, however, must be taken into account if the reservoir is to fulfil its role.

The storage capacity of a reservoir site is limited by topographical considerations and the height of the dam wall. Field surveys can provide data on the reservoir area in relation to the elevation of the water surface at the dam site, from which elevation-storage or capacity curves can be drawn.

If a choice of suitable dam sites is available, it is preferable to choose sites which minimise the surface area of the reservoir in relation to volume, as pointed out in the previous section. These will have steep-sided valleys which allow deeper water levels per increment of storage. Flat terrain makes it difficult to avoid large surface area to volume ratios and, where the cost of providing the water supply is high, it may be economic to consider methods for reducing the evaporative loss.

Demand is clearly related to the livestock population and the consumption by the pastoralists. Under certain circumstances the capacity of the reservoir may be limited to provide water only while there is ample forage. A reservoir can be used as a management tool, therefore, to maintain a balance between water supply and grazing potential.

Given the demand and the capacity of the reservoir, simple storage yield calculations can be made from a knowledge of streamflow and potential evaporation. It should be borne in mind that small reservoirs in the middle of hot, dry areas suffer from the 'oasis' effect. This is the increase of actual evaporation beyond the potential rate due to the addition of a large component of advective heat to the energy budget.

If records of streamflow are available over a sufficiently long period, cumulative streamflow minus evaporation and percolation losses can be plotted against time in a mass curve or Rippl diagram (Linsley and Franzini, 1964). Demand curves representing a uniform rate of demand are straight lines and can be plotted tangentially to a high point of a mass curve (i. e. when the reservoir is full). These represent the rate of withdrawal from the reservoir, and the maximum departure of the mass curve from the demand line represents the reservoir capacity required to satisfy the demand (Figure 27).

The disadvantages of this method are:

i) the analysis is based solely on a historical record which may not include an adequate range of wet and dry conditions;

ii) there is no means of assessing the risk of water shortage, particularly if the run of the record is short; and

iii) the optimum solutions are sensitive to the initial state of the storage system.

An alternative solution is to generate sequences of streamflow from an extended time series of rainfall. This will allow the probability of failure of the water supply to be assessed. Such factors as decreasing storage due to sedimentation and varying demand can easily be assimilated into this type of analysis (Carr and Underhill, 1974).

With the type of small reservoir designed for livestock watering in the semi-arid tropics, the limiting factors will almost certainly be high evaporation during the period of maximum demand and unreliable rainfall to replenish the reservoir. Under these conditions the use of techniques to suppress evaporation, such as those described in UNITAR (1982), may be considered worthwhile. These include the use of monomolecular films to introduce a diffusion barrier between water and atmosphere (see also Mansfield, 1959), the use of floating vapour barriers such as wax-impregnated, expanded polystyrene, and the use of compartmented reservoirs.

Compartmented reservoirs are designed to reduce evaporation by concentrating water into a number of deep compartments, rather than allowing it to spread over a large surface area. Pumps are required to transfer the water from one compartment to another as the storage decreases (Figure 28). It is reported that portable, high-capacity pumps make this method economical for small reservoirs (UNITAR, 1982), although this needs to be tested under African conditions.

Another method of evaporation control is to use sand rivers as storage reservoirs. This technique has been used successfully in East Africa (Ministry of Agriculture, 1962) where the reservoirs are known as subsurface dams. The technique depends for its effectiveness on the availability of an extensive bed of coarse sand of the kind normally found in Precambrian basement areas. By constructing a low weir at a convenient rock bar, coarse particles can be encouraged to settle upstream of the weir. If the height of the weir is raised after successive floods, fine particles tend to be carried over the weir and a deep bed of coarse sand can build up. An abstraction pipe can be built into the lower stages of the weir with a suitably graded filter, rather like the screened casing and gravel pack of a borehole. With careful construction, such subsurface dams can function without trouble for many years. The storage capacity of the reservoir is limited by the void ratio of the coarse sand matrix, however, and its maximum value will range between 30 and 50% under favourable conditions. Well chosen sites can drain quite considerable stretches of river, albeit slowly, and this method can sustain small livestock populations throughout the dry season. No particular skill is required in constructing the low weir, and pipes can be laid to cattle troughs, incorporating valves or taps to control wastage.

The useful life of a normal surface water reservoir in tropical Africa is critically dependent upon the degree of soil erosion in the catchment. Heavy silt loads from eroding catchments can significantly reduce the storage available within the space of few years, and often in one wet season. Degrading catchments also change their storm runoff characteristics, so that flash floods become more frequent and more destructive. This can lead to overtopping and breaching of dam walls. Stromquist (1981) reports a number of reservoirs in Tanzania which have either lost a large proportion of their storage (e.g. 47.5% and 40.7%) or their embankments have been breached due to overcultivation, overgrazing and a high rate of firewood extraction.

Figure 27. A mass curve or Rippl diagram.

Figure 28. Operation of a three-compartment reservoir. Source: UNITAR (1982).

If surface water reservoirs are contemplated, not only will the problems of reliability and sedimentation have to be faced, but also the pollution and health risks referred to in Chapter 5.

6.3 Groundwater exploitation

6.3.1 Dug wells
6.3.2 Boreholes

The occurrence of groundwater has been discussed briefly in Chapters 3 and 4. If an area proves suitable for exploiting the groundwater resource, deduced either from surface and subsurface exploration or from the prior existence of shallow wells or boreholes, the most important criterion determining how the resource should be exploited is depth to static water level.

If the water level is less than 10 m from the surface, it will be relatively cheap and quick to dig a well by hand. Such wells are known to have been dug to great depths (e.g. Wagner and Lanoix, 1959, report depths of over 120 m). The relative costs of a lined or partially lined well dug to more than 20 m and a low-cost borehole of the type described below, however, mitigate against the hand-dug well, if drilling rigs are available. If they are not, hand-dug wells will continue to be constructed to considerable depths, even though they may not represent the most economical option.

One advantage of a dug well over a borehole is that community participation is assured from the start. Self-help labour is usually used to dig the well, and women and children can all help with the fetching and carrying of sand and gravel. A rural community thus identifies itself with the construction of the dug well, and this sense of communal ownership is vital if the water point is to continue to function.

Recently groundwater supply projects have started in Malawi, which integrate dug-well construction with low-cost borehole construction. The self-help concept, known to work successfully with well digging, is developed as far as possible with borehole drilling, and often a rural water supply will consist of a mixture of boreholes and dug wells, all constructed as part of the same programme. In this way community participation in siting, construction, completion and maintenance is encouraged, so that village-level operation and maintenance becomes a possibility.

The method of exploiting a groundwater resource will also depend on the facilities and funds available. The 'United Nations International Drinking Water Supply and Sanitation Decade' (IDWSSD) has focused attention on the plight of most of the rural population of tropical Africa. The very size of the task of providing the bulk of the rural population with safe but untreated water, within a short walking distance, makes it imperative that low-cost solutions be sought. With very few exceptions the cheapest per caput constructions will be dug wells and low-cost boreholes. The exceptions are when aquifers are deep lying, necessitating large drilling rigs and expensive boreholes.

The same principles apply equally well to water supplies for livestock. So many water points are required over such a large area that low-cost solutions are essential. The critical difference lies in the fact that with livestock enterprises some rate of return on investment is envisaged and, therefore, the more expensive options may be justified. In the case of village or community water supplies, no income is generated from the sale of water in the majority of rural areas in tropical Africa. Investment in water supplies for rural communities, therefore, has to be set against indirect benefits such as improvements in health and the release of women's time now spent in water collecting, so that the per caput cost needs to be kept as low as possible.

For this reason, this report concentrates on recent developments in low-cost technology applied to rural water supplies. This is a rapidly changing field stimulated by the IDWSSD, and the range of options will undoubtedly multiply as the decade progresses. The more expensive options centring around high-yielding boreholes, possibly motorised and with a reticulation system, will require professional advice on location, design, construction and development. These topics will be dealt with in outline.

6.3.1 Dug wells

Hand-dug wells are one of the oldest means of water supply. Begun as simple water holes in sand rivers, the concept of finding water by digging in riparian areas has spread away from the river course itself and, particularly in West Africa, deep hand-dug wells, reaching up to 100 m in depth, are used to tap deeper shallow aquifers and areas of basal seepage around inselbergs and escarpments.

Some loss of skills and knowledge of techniques for exploiting traditional sources seems to have occurred recently. UNESCO have a major regional project designed to identify and document traditional techniques for water resources development, which were known to have been successfully exploited within certain areas (UNESCO, 1982). The aim of the project is to transfer such appropriate technology within the sub-Saharan African countries, so that communities can adopt and extend the techniques with a minimum of outside assistance.

Improvements in well-digging techniques are mainly aimed at making the work easier and safer and, at the same time, improving the sanitary completion of the well to prevent pollution. Open wells afford very little protection against pollution, even when low parapets are constructed to prevent the ingress of surface water. Any water-lifting techniques involving the introduction into the well of ropes and buckets which are handled or have been exposed to contamination will create a possible source of faecal pollution. This risk has to be balanced against the extra cost of sanitary protection. The degree of protection, therefore, will depend largely on the individual circumstances of a particular water point. More sophisticated lifting devices, such as hand pumps, will require maintenance of one sort or another. If no maintenance facilities can be provided because of remoteness or lack of sedentary populations, open wells will be a better option. If, on the other hand, the hand pump is suitable for village or community level maintenance, and the frequency of use of the water point is high, it is preferable to aim for a fully protected well.

The simplest type of hand-dug well, therefore, is a wide (> 1 m in diameter) circular hole, dug as far as possible beneath the static water level. It is desirable to protect the sides of the well from collapse, and a common technique is to either sink pre-cast concrete rings or to mould rings in situ. These rings clearly determine the diameter of the well and they are usually designed to allow one or two men enough room to work inside them (i.e. at least 1.3 m in diameter).

Where the geological conditions allow freestanding well sides which are not likely to collapse, an alternative is to confine the lined portion of the well to the bottom of the hole, extending to about 1 m below the water table. This allows the hole to be backfilled on top of a concrete slab, which seals off the reservoir. This type of well must have a hand pump, and two serious disadvantages are immediately apparent: if the hand pump breaks down, there is no alternative means of abstracting water; and if the water level drops significantly in drought years, there is no easy way of deepening the hole.

Undoubtedly, the depth of lining will be a major cost factor in the construction of the well. More experimentation is needed with alternative materials, such as brick lining or sisal cement, to minimise the cost. This is particularly relevant in areas where dug wells exceed 20 m in depth.

Basically, the dug well provides a reservoir of groundwater which can be exploited by pumping or lifting with buckets. The capacity of this reservoir must be sufficient to meet the needs of the community. Normally, intermittent abstraction will allow some recovery (especially overnight) and, provided the storage is sufficient to meet peak demands, dug wells can perform successfully in areas of low permeability. The diameter and the depth of the well have little effect on its potential 'yield' with reference to the aquifer. They do determine the storage capacity, however, and the peak demand must be calculated in terms of the human and livestock populations to be served and the duration of abstraction.

The reservoir is usually constructed of porous concrete rings set below the static water level, with either non-porous rings, brickwork or backfilled material above. The depth below the water table to which rings can be set will depend on how effectively the well can be drained to allow digging. If small, dewatering motor pumps are available, this will considerably ease the enlargement of the reservoir. There is a practical limit to the depth from which a suction pump can lift water, and unless exhaust facilities are provided, motor pumps producing carbon monoxide should not be placed down the well. Electrical, submersible sludge pumps are very suitable for this task, but they require a generator and are generally not so readily available. Dewatering by hand is possible, but it is laborious and slow. It usually results in the reservoir not being constructed deep enough, and may result in failure of the water supply in drier-than-average years.

For this reason, well digging should take place at the end of the dry season when static water levels are at their lowest. The depth and diameter of the storage reservoir should be chosen to provide over half the daily requirement, and preferably nearer two thirds, assuming a 12-hour pumping day. A reservoir 3 m deep and 1 m in diameter, for example, will have a storage capacity of 2356 litres.

Some indication of the potential yield of a shallow well can be obtained by simple pumping tests. To carry out such tests, a means of measuring the water level, a pump or system of rapidly removing water by buckets and, preferably, a measuring trough with a 'v'-notch outflow are required. Water is abstracted at a rate such that the static water level is depressed to the minimum position. This position should correspond to where the intake of a hand pump would be placed, or to the minimum depth in the reservoir which allows water to be abstracted by bucket. The rate of abstraction is adjusted so that the water level remains constant at this position. It can then be measured from the flow over the 'v'-notch in the measuring trough. In areas of low permeability it may be impossible to obtain a continuous rate of pumping low enough to assess the aquifer. In this case buckets will have to be used to estimate the maximum abstraction rate in relation to aquifer yield. In areas of high permeability, on the other hand, it may be impossible to draw down the static water level with the pump available. In this case, however, there is rarely any concern that the yield will be insufficient.

In low-yielding wells an approximation of yield can be made by observing the rate of recovery to static water level. The well is bailed dry and measurements of the water level are taken every minute until static water level is restored. The rise of water level and the times must be carefully recorded. It will be seen that recovery is rapid at first, gradually slowing down to zero as the static level is reached, at which point the pressure in the aquifer is balanced by atmospheric pressure. The volume of the well per unit depth being known, the number of litres recovered and minutes elapsed can be tabulated or plotted as a graph, which will show a flattening curve (Figure 29). An approximate yield can then be determined by averaging out the values in the lower two thirds of the curve, or in the first two thirds of the table, neglecting the flatter portion where recovery asymptotically approaches static level.

Many different designs of protected dug wells can be found in the literature (e.g. Wagner and Lanoix, 1959; US Public Health Service, 1950; UNDP/Malawi Government, 1982; DHV, 1979), and two examples of contrasting design are given in Figures 30 and 31. The advantage of the completely lined well is that even if a hand pump is fitted, it can be provided with a removable manhole cover to allow buckets to be used in the event of a pump breakdown. It is also possible to deepen an open or fully lined well in times of drought.

In completing the well at the surface, whether it is left open or covered with a slab and a pump, it is important to seal or grout the top of the well to prevent contaminated surface water from infiltrating down to the water table. This is particularly important if porous linings such as clay bricks are used in the upper part of the well. A minimum of 3 m of grouting is recommended by Wagner and Lanoix (1959). In very porous subsoils not only will careful attention have to be paid to this aspect, but also the well will have to be sited away from sources of organic pollution (see Chapter 5). It is also desirable to raise the well wall above the level of the ground surface. Open wells should have a wall high enough to remove the risk of people overbalancing and falling into the well (> 50 cm). If a pump is provided, it is good practice to extend the outlet pipe to 3 m or more from the slab. This will help prevent waste water infiltrating back into the well.

The standard practice in Malawi (UNDP/Malawi Government, 1982), where a hand pump is fitted to either a dug well or a borehole, is to construct concrete aprons at the end of an extended outlet pipe and drainage channels to lead waste water even further away from the water point (Figure 30).

An alternative to the wide-diameter shallow well is the driven well point. Its application is limited to shallow sandy aquifers, normally not more than 10 m below the surface. Being of small diameter, usually a nominal 50 mm or 100 mm, they can only accommodate a small pump with a low rate of delivery. They can serve a limited number of livestock, however, and can be a very useful adjunct to an open watering hole, such as a hafir or swamp, to provide relatively clean and safe drinking water to the stock owners.

The well consists of a length of perforated steel pipe, around which wire gauze has been wrapped or, preferably, bronze screening of fine mesh. The leading end terminates in a steel cone which should be of slightly larger diameter than the pipe. The upper end is connected to successive lengths of screwed steel piping to form a column which is driven into the ground by vertical blows of a hammer. The hammer may be worked by hand or suspended from a pulley. With small diameters of less than 75 mm, the water is abstracted by a suction pump coupled directly to the protruding pipe. Larger diameters of 75 to 100 mm will accommodate a 50 mm cylinder with a plunger connected by well rods to a conventional hand pump. The cylinder can draw water from depths greater than the suction limit of 7 m.

6.3.2 Boreholes

Boreholes are also termed 'wells' in American literature and are referred to as 'tube wells' in the Indian subcontinent. They are small-diameter (100 to 120 mm) drilled holes, constructed by mechanical means to depths ranging from 20 to 1000 m. The diameter of the hole is related to the type of pump to be used and to the potential yield of the aquifer. Large-diameter boreholes are usually motorised and constructed in high-yielding aquifers. Their high cost usually only justifies their use for urban or pert-urban water supplies and for irrigation schemes.

Assuming that the most important criterion in providing water supplies for livestock is minimising cost, it follows that borehole diameters and depth must be kept to a minimum. For a given livestock population and given aquifer conditions, the optimum solution may be either a series of small, low-yielding boreholes or a single high-yielding borehole. The exact combination or configuration will depend on individual circumstances.

Hand-pump cylinders generally require an internal diameter of 100 mm. If the borehole is drilled in unconsolidated or partially consolidated formations, a gravel pack will be needed to prevent the ingress of fine particles which tend to cause excessive wear on pump parts or silting-up of the borehole. The gravel envelope should be a minimum thickness of 50 mm, and this requires that the drilled hole be at least 200 mm in diameter. If the borehole is drilled in hard rock, no gravel pack is necessary. In this case the hole diameter can be reduced to 100 mm.

Figure 29. Typical recovery test of a shallow well with a volume of 8 l/cm depth.

With regard to depth, the general principle to follow is that the borehole should be no deeper than necessary. It follows that after water is struck, a decision has to be made when to stop drilling. If a hydrogeologist is supervising the borehole programme, a decision will be taken based on the aquifer thickness and permeability. Bailer tests at 3 m intervals can give an indication of the potential yield as drilling progresses. The hydrogeologist has to make a judgement based on the indicated yield and on the possibility of seasonal or annual fluctuations in water level effecting the adequacy of saturated aquifer thickness.

In the weathered basement rocks, 10 to 15 m of saturated aquifer have usually been sufficient to give yields of 0.5 l/sec or more (UNDP/Malawi Government, 1982). Dijon (1971), in dealing generally with the Precambrian crystalline basement in Africa, also recommends the less clayey, weathered horizons as being the best yielding aquifers and counsels against drilling into fractured rock. He suggests 1 to 1.5 l/sec as being a good yield to aim at in this type of formation. Bannerman (1973) reports more variable yields (0.1 to 1.6 l/sec) from the weathered profile of igneous and metamorphic rocks in Ghana, where the presence of 'kaolinic porridge' or 'flowing arena' complicates construction.

The Kenya Master Water Plan (TAMS, 1979) quotes median yields from the three major categories of rock formations as being 0.7 l/sec for crystalline basement rocks, 1.1 l/sec for sediments and 1.6 l/sec for volcanics. Median specific capacities are highest in the sediments (10 l/min/m) and significantly lower in the volcanics (3.3 l/min/m) and basement rocks (1.6 l/min/m). This gives an indication of the average aquifer thicknesses which are needed to give a yield of 0.5 l/sec: sediments 3 m, volcanics 9 m and basement rocks 20 m.

The above figures must be used with caution because yield and specific capacity are functions of both aquifer characteristics and borehole design. Many of the older boreholes (on which the statistics are based) were poorly designed, and often poorly constructed, in that they 'cased-out' the high-yielding horizons. This is particularly true of boreholes drilled into crystalline rocks of the Precambrian basement. Unless the borehole strikes a fissure, the active aquifer is passed through and unslotted casing is used for most of the water-yielding strata. Recharge is solely dependent on infiltration from the weathered zone down the outside of the borehole casing, resulting in very poor yields.

Borehole design. A good borehole for rural water supplies based on hand pumps should give a sand-free sustainable yield of not less than 0.25 l/sec and preferably 0.5 l/sec. The conventional method of ensuring sand-free water in unconsolidated sediments is to place a gravel pack around the slotted screen which allows water to pass from the aquifer to the pump. The purpose of the gravel pack is to prevent small particles from migrating with the flow of water through the slotted screen.

Figure 30. Example of a back-filled dug well.

Figure 31. Example of a fully lined dug well. Source: Wagner and Lanoix (1969).

It should, ideally, be carefully chosen so that the particle size distribution in the gravel pack gives good permeability, but the pores are too small to allow the passage of the majority of the grains in the aquifer. The ratio of median grain size of pack and aquifer is termed the 'pack-aquifer ratio' (PA) and the optimum P-A ratio will depend largely on the degree of sorting of both materials. Conservative P-A values of 5 to 8 are recommended by UNDP/Malawi Government (1982). Thus, for the median grain size of 0.2 to 0.3 mm found in the weathered crystalline formations in Malawi, a gravel pack with a median grain size of 1.0 to 2.4 mm has been chosen. Coarse quartz sand, which occurs as well graded beach sand at certain sites around Lake Malawi, has been found to be an almost ideal gravel pack.

The common practice of using crushed road-stone chippings of 6 to 12 mm in size is to be avoided since these will not act as a properly graded gravel pack in excluding sand particles from the borehole. This will lead to excessive wear of pump parts and high maintenance costs. The generally accepted range of gravel pack thickness is 75 to 150 mm. If well graded sands are available, of the correct median grain size, the diameter may be reduced to 50 mm to minimise costs.

Once the median grain size of the gravel pack is established, the screen slot size can be chosen. Expensive stainless steel screens with a variety of slot sizes are available on the market. Cheaper materials have been used successfully (GRP, ABS and PVC) even where permeabilities are low. Slotted PVC casing (110 mm Class 10 pipe) is used extensively for all low-cost boreholes in Malawi. The slot size and open area, i.e. percentage of open slot area to the total area of the screen, are the important criteria. Slot size should be chosen to minimise the passage of the graded sand gravel pack through the slots, and a size equal to the d₁₀ of the sieve analysis (i.e. where 90% of the graded sand cannot pass through the slot) will be adequate. With the slotted PVC casing used in Malawi the d₁₀ of the gravel pack is 1 mm and the slot size is 0.75 mm, giving a further margin for safety.

Open area should be matched as closely as possible to the effective porosity of the gravel pack and the aquifer. To reduce the flow velocity through the slots, which will assist in stabilising small particle movement, sufficient length of slotted casing should be used to give the required yield in relation to the aquifer permeability and desired maximum entrance velocities. Open areas of 8% have been achieved with locally manufactured, slotted PVC casing in Malawi. Typical analyses using yields of 0.5 l/sec and maximum entrance velocities of 1.5 cm/sec show that 1.5 m of slotted casing are adequate in the weathered crystalline formation in Malawi (UNDP/Malawi Government, 1982).

The above guidelines indicate that the torch-cut or hacksaw-cut steel slotted casing, commonly used in developing countries, has a completely inadequate open area (frequently less than 1%), and torch-cut slots in particular are much too wide (3 mm). Boreholes constructed with this type of slotted casing and often with the wrongly graded gravel pack have low yields and are very susceptible to sand incursion. The high construction costs of boreholes are only justified if their design is matched to the aquifer characteristics. Poorly designed boreholes may well silt up within a very short time and, at the very least, will require constant and expensive maintenance.

Construction. The method of construction of a borehole will depend upon the depth and diameter required, the nature of the geological formation to be penetrated, and the amount of backup support available. Table 17 gives an indication of performance of the four major methods in relation to the cost of operation and degree of skilled labour required. If minimising costs is the most important factor in providing water supplies, the smaller rigs, although very much slower in operation, are to be preferred to the large and sophisticated multipurpose rotary rigs. If, on the other hand, a large number of boreholes are to be drilled to considerable depths in semiconsolidated or consolidated formations, speed of drilling may become an important factor and the most cost-effective solution may well be to use the more expensive rig. One important point, however, must be borne in mind. Rotary rigs require not only skilled operators, who are in very short supply in developing countries, but also sophisticated repair and maintenance facilities. The simpler rigs can often be repaired without recourse to such facilities, and their longevity and actual rate of production (number of boreholes per year) compare very favourably with rotary rigs.

Hand drilling is the cheapest and simplest form of percussion drilling. This method can be successfully used for boreholes up to 15 m in unconsolidated formations. In Kenya hand drilling with simple percussion rigs originally designed for exploratory work achieved rates of drilling of 5 m per day in unconsolidated material (Ministry of Agriculture, 1962). In the Morogoro area of Tanzania a simple, hand-operated, rotary system has been developed successfully and used to construct boreholes up to 20 m in depth (DHV, 1979).

Table 17. Comparison of different drilling methods.

Source: UNDP/Malawi Government (1982).

For most general purposes the hand-drilling methods are too slow and too limited. Cable-tool drilling rigs use motor power to raise the chisel tools which drill with a percussion action. Many different sizes and designs are available. The smaller rigs are very manoeuverable and much easier to position at sites which have difficult access. They are not always satisfactory, however, when obstructions are met in the drilling operation for which heavier tools are required. The larger percussion rigs are more than adequate for low-cost boreholes drilled into the weathered crystalline formations, and have heavier tools which can be used in more consolidated formations. Most percussion rigs of this type are truck- or trailer-mounted and they are able to penetrate difficult terrain where the much heavier and larger rotary rigs are unable to go.

The drilling operation consists of raising and lowering a string of tools suspended from the drilling cable (Figure 32). Water is added to the hole above the water table and the cuttings from the bit are removed periodically by a bailer. In soft ground the hole is cased with flush-jointed drilling casing to prevent the hole from collapsing. The casing is allowed to fall under its own weight, or it can be driven down as drilling progresses. When the hole has reached the desired depth, slotted and plain borehole casings can be inserted and centred within the drilling casing. The gravel pack is then poured and the drill casing can be pulled out.

The 'hydraulic rotary method' uses a rotating bit with hardened cutting edges attached to hollow tubes through which drilling mud is forced to the working face in the hole (Figure 33). Drilling muds and fluids are special clays, such as bentonite, which give a range of viscosity and specific gravity. Pumped under pressure from the surface, they convey the cuttings through the annular space between the tubes and the sides of the hole to the surface, where they are settled out in a series of ponds. The mud is then recirculated down the hole. The drilling operation has to be continuous to keep the mud constantly in motion. Drilling bits are of many types and hardnesses, designed to deal with all kinds of ground formation and rock.

Casing is not normally required during the rotary drilling process because the circulating mud keeps the hole open. Casing is inserted when the drilling is completed, according to the needs of the formation. This is normally done for the first part of the hole (when it is in soft ground) down to the firm rock, below which casing should not be necessary.

Rotary hydraulic rigs are very fast in soft ground with an output of 100 m or more per 24 hours. They are also very fast in hard rock if the correct bit is used. Hard rock bits wear quickly, however, and are costly to replace.

The equipment of a rotary rig, which is commonly mounted on a self-propelled six-wheeled vehicle, comprises a prime mover, normally a diesel engine of 100 to 200 hp. a hydraulic pump for actuating the various controls and the rotary movement of the drill stem, and a high-capacity centrifugal pump for mud circulation. Ancillary equipment such as an electric lighting plant, welding sets, grinders, water trailers or tankers and fuel bowsers make up a formidable complement which, in total, may weigh some 50 t. Capital investment of this magnitude can be justified if one remembers that such a rig can finish a 200 m hole in less than a week. Profitability, however, will depend on continuous employment of the equipment.

The 'reverse circulation method' uses the same type of rig as the hydraulic rotary method, but the principle is different. Here the cuttings are removed by water drawn up through the drilling tubes by a suction pump at the surface. This requires a constant head of water to be kept above the rest water level of an aquifer. The hydrostatic pressure keeps the sides of the hole open, and this is assisted by fine particles of clay and sand from the cuttings, which are returned with the circulating water and which line the sides. It is essential to keep the hole full of water, as failure of the hydrostatic pressure could cause the collapse of the sides and the loss of expensive tools. This method is most suitable for drilling large-diameter holes (up to 1200 mm) in unconsolidated formations.

The 'down-the-hole hammer' combines the features of percussion drilling with those of rotary drilling, using compressed air to drive a rotating pneumatic hammer at the end of the string of tools. Exhaust air from the hammer, issuing from ports, travels at high velocity (1000 m/min) and removes the cuttings. This type of rig is smaller than a rotary rig and more compact. It is normally mounted on a self-propelled chassis and energy is supplied by a powerful compressor driven by a diesel engine of up to 200 hp. Compressed air is used to drive various motors for raising and lowering the drilling string as well as operating the hammer, and must be supplied at a pressure of between 100 and 200 p.s.i. (or 7 to 14 kg/cm^-2).

The down-the-hole hammer method of drilling for water has been developed from the technique of drilling blast holes in quarries using light compressed air rigs. The original concept was to drill small-diameter holes rapidly in hard rock. This was successful when quarrying in crystalline formations, but proved a failure in drilling through soft formations, largely because the supply of air was insufficient to compensate for losses in the loose ground. On reaching the water table in a sandy aquifer a stoppage of even a few minutes, which interrupts air flow, can result in clogging of the hammer which then will have to be stripped with special tools. In remote areas, if the tools and the skill to effect the repair are lacking, this can mean the loss of many working days.

Figure 32. Diagram of the cable-tool (percussion) method of drilling.

Figure 33. Major components of the rotary method of drilling.

Conventional down-the-hole hammer rigs, specifically designed for quarry work, are normally unsuited to water boring, but development of the principle over the past two decades has produced equipment which is well able to deal with aquifer conditions in hard ground. The down-the-hole hammer is faster in hard rock than the percussion or rotary bit. Improvements in hammer design and, above all, the provision of an adequate compressed air capability, have established the method in the field of groundwater development. Deep-seated aquifers underlying thick flows of fresh lava can be reached quickly and economically. For this reason manufacturers of rotary equipment include a large compressor and the hammer, tools and fittings in the complement of their units. A combination of rotary and down-the-hole techniques is often the answer to difficult subsurface conditions.

Development. After a borehole is constructed, it is 'developed' to stabilise the gravel pack and the formation immediately adjacent to the borehole. Development involves the creation of much higher water entry velocities into the borehole than would normally be encountered during pumping at the design discharge, together with 'backwashing' into the formation. It causes fine particles to move into the hole and encourages the rearrangement of the remaining particles into a more stable matrix. During normal pumping following development, therefore, the water is much cleaner.

Various development techniques can be used, such as surging with compressed air and surge blocks, and 'rawhiding' or overpumping. The choice of technique will depend to some extent on the type of slotted casing employed. PVC casing, for example, is vulnerable to damage when bailing is used to remove the fine silt settling at the bottom of a borehole. For this reason overpumping is usually preferred to surging in PVC-cased boreholes.

With well designed and properly constructed boreholes, overpumping is rarely required for more than 5 to 10 hours to obtain a sand-free discharge. If a high proportion of fine particles is encountered in the formations adjacent to the screen, this time will have to be extended until clear water is being pumped.

Borehole test pumping. After development, test pumping is usually carried out to provide information on the hydraulic characteristics or efficiency of the borehole and on the hydrogeological characteristics of the aquifer.

Data obtained from test pumping can be used to determine the specific capacity or the discharge/drawdown ratio. This gives a measure of the efficiency or productive capacity of the borehole and indicates at what level the pump should be set.

Test pumping for relatively low-yielding boreholes designed for hand pumps need only be carried out to ensure that the borehole is able to sustain an adequate yield. Aquifer tests and more elaborate test pumping are expensive and time-consuming, and are normally only carried out in a major development of an aquifer.

The simple tests should preferably be carried out at a rate higher than the maximum delivery of a hand pump, and continue for at least 4 hours. This will ensure that there is no danger of overpumping or pumping air during normal operation.

Under no circumstances should boreholes designed for hand pumps be motorised at a subsequent date without professional advice. Increasing the discharge of boreholes beyond their design capacity can cause irretrievable damage and often complete collapse.

6.4 Water lifting devices

6.4.1 Selection of pumps
6.4.2 Power for pumping
6.4.3 Wind energy technology
6.4.4 Solar energy

Unless water is supplied under hydrostatic pressure from a surface source above the water point or from an artesian basin, a means of raising or lifting the water is required. From the earliest times man has found it necessary to raise water from a source to supply the needs of himself, his animals and his crops, and throughout the world much ingenuity has teen shown in devising appliances to make the task easier and quicker.

Beginning with the wood or leather bucket to bail water from a water hole, a stream or a furrow, the shaduf and the Archimedes' screw were developed and are still in use, as are many devices using animal power. In nomadic societies, water is still being raised from wells by leather buckets and ropes, or by a team of several men standing one above the other and passing the water upwards in small containers. To anyone who has observed this operation used to water a large herd of cattle the hourly output is impressive, and is witness to the energy which men are prepared to expend to safeguard their herds. Such traditionally developed methods, however primitive, are adapted to the situation and are an appropriate technology, if not always the most efficient. In contrast the reluctance of these same people, who are willing to expend great manual effort, to contribute money for the operation and maintenance of modern mechanical equipment is manifest in many instances, even if the money is there and the service is generally appreciated.

The appropriateness of any mechanical device is determined not only by its reliability and simplicity in operation, but also by its acceptance by the users. For example, it has been the East African experience in pastoral areas that attempts to replace the bucket and rope, or chain of men, with a hand pump, which allows the top of the well to be sealed and the water kept free from pollution, have not been successful because it was claimed by the users that the hand pump was more energy consuming and had a lower output. Hand pumps fitted on wells were neglected, frequently vandalised or abused to the point of breaking the surface equipment. They were soon discredited and abandoned.

In such circumstances some protection of the well was achieved by building a coping extending from some 2 m below the surface to some 60 cm above ground level. Capping it with a concrete lid allows a manhole to be installed, sufficiently large to allow the traditional bucket to be lowered should the hand pump fail. Cattle troughs arranged around the circumference were also a welcome improvement.

The hand pump has found acceptance in those areas of the tropics where the supply is mainly intended for human consumption, and where only small numbers of stock have to be watered. Much research has been done on devising a pump which would be cheap, robust and easy to maintain. Work still continues in several African countries and on a global scale with the UNDP/World Bank Rural Water Supply Hand Pumps Project. A number of good patterns of hand pumps have been developed and recent developments, particularly in Malawi, show that simple, robust hand pumps can be locally produced in developing countries, with a minimum of imported parts (UNDP/Malawi Government, 1982).

In countries where people live in quasi-nucleated communities it is possible to provide a communal water supply either by direct grant or by subsidy, and hand pumps fitted on wells or low-cost boreholes within easy walking distance (less than 1 km) are often the least-cost solution. The problem of maintenance still remains. Moving parts must be regularly greased and cleaned and wearing parts must be replaced from time to time, under normal working conditions. With the new types of village-level operation and maintenance hand pumps, maintenance becomes the responsibility of the community, but unless there is an organisation to set aside funds, however small, to employ a trained man or woman and to provide the parts and materials, breakdowns in the supply inevitably occur.

In recent times the development of water supplies for livestock and other uses has tended to rely heavily on the conventional pump imported from the developed countries at substantial cost. A wide variety of pumps suited to any given situation is available (Table 18). Where the supply is motorised, maintenance problems increase and, even in areas that are not so remote, reliability is a function of efficiency in supplying stocks of fuel and spare parts. If the supply should break down because of lack of transport over poor roads or because of failure to provide the necessary funds at the right time, much hardship will result, and stock may have to be moved to other, better watered areas, thus upsetting the grazing rotation pattern and further endangering the pasture.

With the rise in the cost of fuel, increasing attention is being paid to the development of alternative energy sources to drive pumps. The aim is to produce techniques which can remain unchanged in concept and design for the foreseeable future. Some of these methods are described below. In the nomadic areas of the arid and semiarid zones, however, reliance on human energy will continue for a long time to come.

Where the keeping of livestock is a part of mixed farming activities, there may not be the human time or energy to spare for pumping by hand unless the water source is on the farm and the animals are few. The demand for easily available water continues to increase, and emphasis is now placed on developing community supplies, which include livestock water, in order to spread the capital cost over many consumers. This presupposes an adequate source which may lie some distance away from the point of use, sometimes as far away as several kilometres.

The simplest communal supply will take the form of an intake at the source, a pumping unit if gravity command is not available, a pipeline, a storage tank to hold a 2 days' supply, a distribution system to standpipes in the village, and a cattle trough away from the residential area.

The choice of the right pump for any particular situation is of the utmost importance. Experience shows that pumping problems are most often responsible for the breakdown or poor operation of small water supply systems. The most common types of pump for small water supply systems are:

i) hand- or power-operated reciprocating pumps with the pump cylinder above the ground;

ii) power-operated centrifugal pumps with the pump mechanism above the ground;

iii) hand-, power- or wind-operated reciprocating deep-well pumps with the pump cylinder in the well;

iv) deep-well turbine pumps, powered either from the surface or from a submersible electric motor;

v) jet pumps, power-driven at the surface; and

vi) hydraulic rams (Wagner and Lanoix, 1959).

Under ideal conditions the pressure of the air at sea level is enough to raise a column of water 10.3 m in a vertical pipe in which a perfect vacuum has been made. In practice the suction lift, i.e. the vertical distance from the pump cylinder to the water surface, is never more than 5 to 6 m. If the suction lift is required to be greater than 5 to 6 m, the pump cylinder should be lowered into the well.

Table 18. Relative merits of pumps for use in small water supply systems.

Source: Wagner and Lanoix (1959).

6.4.1 Selection of pumps

It is not possible to establish strict rules for selecting pumps. The most important considerations are:

i) capacity and lift required;
ii) initial cost of the pump and its driving equipment;
iii) cost of operation;
iv) the extent and reliability of the service which will be available for maintenance; and
v) the desirability of standardisation and reduction of the number and diversity of spare parts.

The following remarks can also be made:

1. Pumps in which all moving parts are above ground and easily accessible are preferable and will, in most instances, give the best service simply because they are easier to maintain with the means available in rural areas.

2. Where motor power is used efficiency is a very important factor, and the more expensive the power, the more important becomes the efficiency.

3. Centrifugal and deep-well turbine pumps must operate under the conditions for which they were designed, or a great loss in efficiency will result. Pumps with flat efficiency curves are to be preferred for rural water systems since they allow for greater flexibility in design.

4. Parts for the selected pump should be easily available and, preferably, manufactured within the country of use.

In selecting an engine, or prime mover as it is commonly known, a number of factors must be taken into consideration, the first being the duty required of it. The starting point is a calculation of the power required to lift a given quantity of water to a given height in unit time. The test pumping of a borehole gives the quantity that may safely be extracted from a given depth, taking into account the permissible drawdown and the depth at which the pump has to be set.

The total lift includes the vertical distance between the bottom of the pump cylinder or intake and the point of delivery at or above the surface, plus the friction loss in the piping which is expressed as a percentage of length (metres per 100 m). The friction factor varies with the diameter of the pipe, the material of which it is made, and the volume of water to be transmitted in unit time. Pipe manufacturers publish tables and graphs of friction loss, calculated for all measurements of pipe size, volume of water and distance.

The power to be supplied by an engine must be greater than the power required to lift the water because of efficiency losses in the transmission and in the pump itself. Other efficiency losses result from increases in altitude and in ambient temperature, compared with those for which an engine is rated.

The efficiency of the transmission system, of the well head of the pump, will vary according to the type and make, and will have to be estimated for each case if power is a critical factor and accuracy is required.

A rule-of-thumb for field use, which has generally been proved reliable, is to multiply power required to lift the water by a factor of 2.5 at the surface, or 3.3 at a depth of 100 m. To arrive at the actual brake horsepower of the engine, the required power thus calculated must be further adjusted by the aerating factors for altitude and ambient temperature. Derating factors are given by the manufacturer for each type of engine. For example air-cooled, diesel engines, such as would be used on a borehole installation, are aerated by 3.5% for every 300 m above sea level and by 2% for every 5.5°C above 30°C.

Having assembled the basic technical data for a borehole installation, including the required power of the prime mover, the designer is faced with the task of selecting the most appropriate type and make. AH mechanical equipment must be robust, easy to operate and to maintain, economical in fuel and lubricants, and reliable. Most important is the ready availability of spare parts and servicing facilities within a reasonable distance. This last factor outweighs any initial financial advantages that may be gained from acquiring low-priced equipment of unproved quality for which servicing facilities do not exist in the country of use.

It has certainly been the East African experience that equipment bought from the developed world by the State or by private individuals, or presented by well-meaning donors, without insisting that the manufacturer at the same time establishes a spares holding and servicing agency, has soon failed. Borehole and other installations have been known to stand idle for months or even years, awaiting the arrival of a small but essential spare part. Many have, in the end, been replaced by other plants supplied by locally established agents, in whose interest it is to ensure that their reputation, and hence their business turnover, is maintained, and that their products and their back-up organisation give good service.

In the livestock areas of the arid and semiarid zones which are remote from markets and other centres, and where boreholes and other sources of surface water or groundwater are exploited by mechanical means, plant failure can have the most serious consequences, directly on the animals and their owners and indirectly on the range potential. A grazing block with forage still intact but with a failed water supply will be underused while, inevitably, a neighbouring block with water will be at risk through overgrazing, either by premature occupancy or by the influx of more animals than it can carry. The operating water supply may also become overstrained. Where the supply is from a well or a borehole, excess pumping hours may damage the plant and also the aquifer, both of which in the end may fail totally. The argument for reliable equipment of good quality, even at extra cost, backed up by a reliable service agency within easy reach, is quite clear.

6.4.2 Power for pumping

In addition to maintenance costs, a major cost of pumping is the cost of power to operate the pump. In areas of limited economic return the efficient use of available power is of utmost importance. The types of power available are as follows:

i) Manpower and animal power are the oldest and, in many places, the only form of power available. Either can be completely adequate and should not be overlooked nor underestimated. From ancient times simple devices have been used for raising water which depend on human labour. More advanced devices using animal power can raise water from riverbeds or wells up to an apparent economic limit of about 9 m (Carruthers and Clark, 1981). Manpower output for water lifting, with primitive pumping, is in the range of up to 10 m³/man-hour. Oxen tire more quickly than men, and a pair of oxen generally work only 5 hours a day. Output can be up to 70 m³/h for a pair of oxen, depending on climate.

ii) Gravity is the cheapest source of power, when it is possible to use it. Considerable extra installation costs are justified for such schemes because of the significantly lower recurrent maintenance costs. Usually an economic formula can be worked out, based on the life and cost of pumping equipment as compared to the cost of a gravity pipeline.

iii) Windpower is another cheap source of power which should be given careful consideration for either individual or community supplies. The rising cost of petroleum has generated a renewed interest in windmills for pumping water wherever conditions are favourable.

iv) Solar energy is another source which is claimed to have low operating costs, a long lifetime of the equipment installed, minimum maintenance problems and no need for skilled manpower (ECA, 1976).

v) Electricity, if available at a reasonable cost, is to be preferred to most other systems because of the low capital and operating costs. Electric motors require very little attention and give long service.

iv) Internal combustion engines using petroleum, diesel or kerosene. These engines, in spite of being expensive and costly to maintain, are the most common types of power source found in the rural areas. Experience in such areas indicates that diesel engines are generally the best option, even though they are the most expensive in original cost (Wagner and Lanoix, 1959).

6.4.3 Wind energy technology

The use of wind power also has a long history. Over the centuries windmills have been used to grind corn, as in Western Europe, and to raise water, as in Crete and in other parts of the Mediterranean. From the early structures of wood and cloth or straw matting, windmills have progressed to structures of steel with elaborate gearing systems aimed at increased efficiency. More recently, wind power has been harnessed to generate electricity, and research continues on the design of windmills that can be made locally and cheaply by small-scale craftsmen without loss of quality.

However efficient a windmill may be, it still depends on the constancy and reliability of the wind. While it may not be critical if corn cannot be ground for 2 or 3 weeks because of a succession of still days, it is very critical when water cannot be pumped or electricity generated. A partial and expensive remedy then lies in the provision of storage - large surface or elevated tanks for water and large banks of batteries for electricity.

Such storage can rarely be fully satisfactory because the duration of a windless period cannot be predicted and there is a risk that the storage may be inadequate. The provision of watertight tanks above ground level represents a substantial capital outlay. A safe alternative is to duplicate the windmill with a fuel-driven pumping unit which would only be used on days with insufficient wind, but this also represents a substantial capital outlay.

In tropical Africa wind energy is not as reliable as in temperate latitudes. The eastern coastline is dominated by the monsoons which blow strongly up to about 10 km from the sea, after which they weaken and tend to be gusty. Between the monsoons there is a windless period which may last for several weeks. In the East African highlands monsoon winds penetrate, but vegetation and topography tend to decrease the surface winds. Even on the open plateaux completely windless periods of up to 6 weeks have been recorded. In hilly country the water source is usually found in the bottom of a valley where steady winds do not penetrate, and under such conditions a windmill would be completely uneconomic.

The plateaux of central tropical Africa, situated west of the Great Rift Valley, are still under the influence of trade winds which blow continuously for long periods, but they are considerably weakened away from the coast and there are still significant seasons of nearly windless days.

On the other hand, there are many areas where sufficiently high and constant wind speeds make wind power a suitable alternative energy source. Maintenance costs must be taken into account, however, and windmills require a degree of supervision to ensure that they are not damaged in periods of very high wind speeds.

Usually a limiting factor in the use of wind energy is the lack of sufficient data on wind speed and duration. Most small climatological or agrometeorology stations only record run-of-wind at 2 m above ground level. These data are insufficient to estimate wind speed and duration at the height of the rotor blades.

All systems use a rotating or oscillating collector to convert the kinetic energy of the wind stream into a primary motion which can be used to perform work. Wind energy conversion is ruled by several basic principles which determine how much can be extracted from the wind and the extraction capabilities of the various types of rotor. When a wind system rotor is placed in the free wind stream it imparts predictable changes in air pressure and velocity. As the wind stream approaches the turbine, the air pressure in the stream increases and the velocity decreases until the flow reaches the rotor plane.

As air moves against, over and through the rotor, it imparts energy to the rotor and loses pressure and speed. Further speed losses occur as the wind stream recedes from the rotor and expands in the rotor wake.

The amount of pressure and velocity change is dictated by the characteristics of the rotor; in particular its size and shape, its ability to extract energy, and the freedom of movement allowed by the systems to which it is connected.

The fact that the flow must be maintained limits the amount of energy which can be extracted. If all energy could be extracted, the wind speed behind the rotor would be zero. A balance must be maintained between the extracted energy and the energy flowing through the rotor. The optimum loss of wind speed which would occur is two thirds of the initial velocity. A 100% efficient system can extract the maximum percentage of wind power, derived from the following formula:

(WMO, 1981)

where

P = power density
A_T = swept area of the turbine
V_o = initial wind velocity
s = ambient air density.

The factor 0.593 is called the Betz limit or Betz coefficient.

Most high-performance systems now in use have extraction efficiencies far below their maximum in order to optimise their performance per unit cost. It is usually cheaper to extend blade length or tower height than to optimise rotor efficiency. The optimum power coefficient is approached when the speed of the blade tip is five to ten times the speed of the wind. For example, wind tunnel tests have shown, for 'ideal' rotor configurations, that maximum power coefficients of 0.42 can be obtained. This is 71% of the maximum efficiency attainable (WMO, 1981).

Major changes in the 'state-of-the-art' for small (less than 100 kW) electrical output wind systems have occurred during the past few years, including:

- the replacement of the upwind rotor with the downwind rotor;

- the emergence of the vertical-axis rotor as a major competitor to horizontal-axis machines; and

- the growing prevalence of a.c. output systems.

An important factor is also the increasing role of governments in supporting the improvement of these systems. Significant developments in mechanical output of the systems have included the production in the USA of 40 kW prototypes for electrical or mechanical output (WMO, 1981). At the same time, conventional 2 to 5 fan-type water pumps are still in production, with sales increasing.

Low technology, wind energy systems have the advantage of being easily built and maintained, using local labour resources. While their performance may be less than that of more sophisticated systems in terms of efficiency, their overall performance per unit cost may well prove superior due to the use of secondhand parts or locally available materials. (Fraenkel, 1975; WMO, 1981; Mamo and Jensen, 1981). The value of using locally available materials and labour resources cannot be overemphasised. Local availability means fewer delays and interruptions associated with obtaining spare parts, maintenance and repair services.

Modern multiblade or vertical-axis windmills have excellent performance characteristics, matching operational capability in low wind speeds (5 km/h) with reliability and minimum field maintenance requirements. Some systems allow the integration of auxilliary power units for periods of no wind.

A vertical-axis rotor design produces a gyroscopic effect which allows whole units to be post-mounted, thus eliminating the need for guide wires or expensive scaffold-type masts. Such wind pumps tend to be structurally simpler, having far fewer moving parts than fan- or propeller-type windmills.

In data-spare regions an interpolation of wind observations may not provide a reliable wind resource analysis. Often the data-sparse regions are in areas of complex terrain, and certain indirect indicators of wind energy, such as topographical situation, vegetation features and aeolian land forms, may aid the analysis.

The first requirement for the design of a wind energy system is to know the average frequency of occurrence of mean wind speed values at rotor height. It is also useful to know the vertical distribution of the mean wind speeds, or wind profile. Thirdly, maximum wind speeds need to be assessed in order to define the aerodynamic loads. Siting without knowing the wind characteristics may lead to less power output than is required.

The cost of wind power should be known to a reasonable accuracy before the decision is taken to install a wind energy system. Such an assessment also requires an accurate knowledge of wind characteristics at the site.

6.4.4 Solar energy

Tropical Africa is richly endowed with potential solar energy. Away from the highlands, which tend to generate cloud even during the dry season, much of the semi-arid zone has between 8 to 10 hours per day of bright sunshine in the dry months.

Considerable progress has been achieved in harnessing solar power, notably in West Africa where quite large solar units have been developed to produce an output of 25 kW and more. Such units require considerable technical skill to install and are still very costly. A unit developing 1 kW costs, at present, over US$ 20 000. As such, it is only justified for human supply in areas where no other cheaper possibility exists.

There are a number of solar water pumps in operation in various parts of the world. The first solar pump in Africa was installed in Dakar, Senegal, in 1968. Most units are barely 1 to 2% efficient and extremely expensive per cubic meter of water pumped, even from shallow wells. Intermittent pumps operated from solar cells have also been tried. The economics of their operation do not compare favourably to conventional energy sources, except in locations of dire need where no alternative source of power is available. The 1977 costs were in the range of US$ 10 000 to 20 000 per kW of generating capacity.

The most promising new option on the horizon for water pumping is the concentrating photo-voltaic system, where costs in the range of US$ 1000 to US$ 2000 per kW were estimated in 1980 (UNITAR, 1982).

Further research into the technology of harnessing solar power continues and is aimed at reducing the capital cost, not only by using cheaper materials and turning to mass production, but also by developing a local capability for manufacturing at least some of the parts, such as the solar cells.